1. Field of the Invention
The present invention relates to a solar cell, and more particularly, to a thin film type solar cell.
2. Discussion of the Related Art
A solar cell with a property of semiconductor converts a light energy into an electric energy.
A structure and principle of the solar cell according to the related art will be briefly explained as follows. The solar cell is formed in a PN junction structure where a positive (P)-type semiconductor makes a junction with a negative (N)-type semiconductor. When solar ray is incident on the solar cell with the PN junction structure, holes (+) and electrons (−) are generated in the semiconductor owing to the energy of the solar ray. By an electric field generated in a PN junction area, the holes (+) are drifted toward the P-type semiconductor and the electrons (−) are drifted toward the N-type semiconductor, whereby an electric power is produced with an occurrence of electric potential.
The solar cell can be largely classified into a wafer type solar cell and a thin film type solar cell.
The wafer type solar cell uses a wafer made of a semiconductor material such as silicon. In the meantime, the thin film type solar cell is manufactured by forming a semiconductor in type of a thin film on a glass substrate.
With respect to efficiency, the wafer type solar cell is better than the thin film type solar cell. However, in the case of the wafer type solar cell, it is difficult to realize a small thickness due to difficulty in performance of the manufacturing process. In addition, the wafer type solar cell uses a high-priced semiconductor substrate, whereby its manufacturing cost is increased.
Even though the thin film type solar cell is inferior in efficiency to the wafer type solar cell, the thin film type solar cell has advantages such as realization of thin profile and use of low-priced material. Accordingly, the thin film type solar cell is suitable for a mass production.
The thin film type solar cell is manufactured by sequential steps of forming a front electrode on a substrate, forming a semiconductor layer on the front electrode, and forming a rear electrode on the semiconductor layer.
Hereinafter, a related art thin film type solar cell will be described with reference to the accompanying drawings.
FIG. 1 is a cross section view illustrating a related art thin film type solar cell.
As shown in FIG. 1, the related art thin film type solar cell comprises a substrate 1; a front electrode layer 2 on the substrate 1; a semiconductor layer 3 on the front electrode layer 2; and a rear electrode layer 7 on the semiconductor layer 3.
The semiconductor layer 3 is formed in a PIN structure where a P (Positive)-type semiconductor layer 4, an I (Intrinsic)-type semiconductor layer 5, and an N(Negative)-type semiconductor layer 6 are deposited in sequence. The semiconductor layer 3 is generally formed of amorphous silicon.
However, the related art thin film type solar cell with the semiconductor layer 3 of the amorphous silicon is problematic in that cell efficiency is lowered due to the increase of degradation rate after the lapse of time. Among the factors leading to the increase of degradation rate is a plurality of dangling bonding sites or Si—H2 bonding sites existing in the semiconductor layer 3. Under the currently-known processing conditions, it is difficult to deposit the amorphous silicon while decreasing the number of dangling bonding sites or Si—H2 bonding sites. Thus, the increase of degradation rate is induced by the plurality of dangling bonding sites or Si—H2 bonding sites existing in the deposited amorphous silicon.
In addition, a deposition rate of the amorphous silicon has to be increased for improvement of the yield. However, the increase of the deposition rate of the amorphous silicon may cause the increase of dangling bonding sites or Si—H2 bonding sites existing in the deposited amorphous silicon. If RF power is applied more so as to increase the deposition rate of the amorphous silicon, the number of dangling bonding sites or Si—H2 bonding sites existing in the deposited amorphous silicon is increased more. Accordingly, it is difficult to improve the yield due to the limits on the increase of the deposition rate of the amorphous silicon.